Optical measuring systems for precisely measuring the three-dimensional surface contour of objects are well known. Such systems commonly optical lasers, such as laser trackers, are cooperatively used with a reflective target. Spherically mounted retroreflectors (SMR) are a commonly used reflective target. Typically, an SMR consists of a retroreflector, such as a corner cube mirror, which is mounted within a steel sphere. The retroreflector is particularly mounted within the sphere such that its vertex is at the center of the sphere. The sphere is provided with an aperture which allows line-of-sight access by the laser to the retroreflector.
The laser tracker is configured to project a laser beam through the aperture at the retroreflector. The laser tracker is further configured to receive a reflected laser beam from the retroreflector. The reflected laser beam is used to determine the location of the vertex of the retroreflector, and therefore the location of the co-located center of the sphere.
In operation, the outer surface of the SMR is placed in contact with the surface contour of the object being measured. The retroreflector is oriented toward the laser tracker. The laser tacker is then used to determine the three-dimensional coordinates of the SMR center. The SMR center is a fixed distance away from the surface contour, which is equal to the radius of the sphere to its outer surface. Thus, the three-dimensional coordinates of the contact point of the SMR sphere on the contour surface can be determined based upon such radial or offset distance. The SMR may be manually moved about the surface contour for mapping additional contact points in order to measure or map the surface contour at intervals of time or distance moved.
The ability to accurately perform measurements is challenging at or near severe changes in the surface contour. In particular, where the object being measured has an edge, measurements taken near the edge present certain difficulties. For example, the object being measured may be a relatively thin sheet structure, such as a panel. The panel is provided with opposing main panel sides which terminate at corner edges. Narrow panel edge surfaces are interposed between the corner edges of the main panel sides. In order to measure the panel edge surfaces using the above described SMR, the operator must attempt to maintain the sphere in contact with them. Where the thickness of the panel edge surfaces is only 0.1 inches, for example, maintaining such contact may be extremely challenging. The reason is that because of the relatively narrow geometry of the panel edge surface, the sphere has a tendency to slide off resulting in disengagement of contact.
A device called a "pin nest" is a prior art attempt to address this problem. The pin nest is used to aid in positioning the SMR when taking measurements of such narrow panel edge surfaces. The pin nest is generally cylindrical shaped. The diameter of a standard pin nest is about one and one half inches. The pin nest has an open end and a closed end which defines a pin nest base. A steel pin protrudes perpendicularly from the pin nest base along the axis of radial symmetry of the pin nest. The pin may be 0.25" in diameter and is defined by a cylindrical outer surface.
In use, the pin nest is placed on the object by resting a portion of the pin nest base upon the panel side with the pin extending over the corner edge. The outer surface of the pin is placed in contact with the panel edge surface to be measured. The SMR is placed in the pin nest by placing the SMR sphere within the open end of the pin nest. Thus, the SMR sphere is cupped by the open end and is centered over the pin. The laser tracker is then used to measure the location of the retroreflector. The pin nest with the SMR is then moved along the corner edge while maintaining contact with the pin to the panel edge surface to take additional measurements.
In theory, the points measured with the laser tracker and the pin nest are at a known location. This is because the pin nest is configured to position the SMR a "fixed" distance above the panel side and a "fixed" distance away from the panel edge surface (equal to the radius of the pin). In practice, however, the accuracy of the measurements relies on several assumptions regarding the geometry of the object being measured. Erroneous measurements will result if the operator does not properly position the pin nest upon the panel side. If the surface that the pin nest sits on is curved, then it is easy for an operator to inadvertently rock the nest. Thus, in using such a pin nest, it is assumed that the panel side that the nest sits on is flat. In addition, as the pin protrudes perpendicularly from the base of the pin nest, it is further assumed that the edge surface is normal to the panel side that the nest sits on. Thus, if the edge surface is not square to the panel side, then the offset introduced by the pin will not be at a "known" location. Moreover, in general, laser tracker measurement accuracy is greatest and there is the least opportunity for error if the SMR is in direct contact with the surface being measured.
Accordingly, there is a need in the art for an improved adapter which can be used with an SMR for taking surface measurements which are adjacent to an edge of an object.